The present invention relates to the field of catalysis, in particular the field of catalyst materials.
In particular, the present invention relates to a process for producing carbon substrates loaded with metal oxides, in particular carbon materials containing metal oxide nanoparticles, which substrates are preferably suitable for use in and/or as catalysts or else in and/or as bipolar plates, and to the actual carbon substrates thus obtainable and the use thereof, particularly in and/or as catalysts or in and/or as bipolar plates.
There are a large number of chemical reactions that only occur in the presence of a catalyst or only result in significant yields of product in the presence of a catalyst.
Generally, the term ‘catalysis’ is understood to be the change or decrease in activation energy and thus the change in the reaction rate of a chemical reaction caused by the participation of a catalyst without changing the thermodynamic equilibrium.
The term ‘catalyst’ denotes a substance that reduces activation energy of the chemical reaction concerned and thus affects the reaction rate of this chemical reaction without being consumed or reacted itself. The catalyst emerges un-changed from the overall reaction and thus can perform a number of catalysis cycles.
Since more than 80% of all industrial chemical products come into contact with catalysts during their production, the value added by these is very high and of considerable economic importance. Catalysts are thus used in more than 80% of all known, industrial chemical processes; without the presence of catalysts the chemical reactions concerned would not take place or, at best, would take place a lot more slowly or incompletely.
‘Heterogeneous catalysts’ are used in a large number of catalysed chemical processes. Reference is made to heterogeneous catalysis if, during a chemical reaction, the catalyst on the one hand and the reacting substances or starting materials on the other are present in different states of matter. By far, the most frequently used state of matter of heterogeneous catalysts is the solid state. The catalyst consists either completely of the active component (‘full catalysts’) or the actual effective active component is applied to a support material, which is generally the case (‘supported catalysts’). For example, a suitable substrate (for example carbon, such as active carbon, aluminium oxide, silicon oxide, etc.) can be loaded with the actual catalytically active component, in particular can be impregnated therewith or similar.
For example, supported catalysts based in metal oxides, such as titanium dioxide, vanadium pentoxide and tungsten oxide thus can be used in a versatile manner, for example in the selective catalytic reduction (also known synonymously as SCR) of nitrogen oxides in exhaust gases of firing systems, waste incinerator plants, gas turbines, industrial plants, engines, etc., nitrogen oxides being selectively reduced whilst undesired secondary reactions such as oxidation of sulphur dioxide to form sulphur trioxide are largely suppressed. For example, the aforementioned SCR process is applied in the automotive industry in order to reduce the pollutant emissions of diesel motor vehicles.
The above-mentioned, titanium dioxide especially, preferably in crystalline form such as rutile and/or anatase, can be used as a heterogeneous catalyst, particularly ‘photocatalysis’. For example, TiO2 nanoparticles are thus implemented for ‘self-cleaning’ surfaces, organic materials on the surfaces concerned being decomposed by irradiation of UV radiation, in such a way that these surfaces remain clean and antimicrobial.
However, the support catalysts known from the prior art pose a range of drawbacks: on the one hand the catalytically active component is often not present in sufficiently small particles so an optimal (specific) surface area is not always available for catalysis. Furthermore, it is often necessary to proceed with an excess of catalytically active component since, when loading the support material, some of the catalytically active component is not freely accessible for catalysis.
A further drawback of the support catalysts known from the prior art is, in particular, that the catalytically active component is not always sufficiently immobilised on the support, in particular, under the often extreme reaction conditions, the catalytically active component migrates on the support surface and thus there is no longer a uniform distribution over the entire support surface; this undesired migration of the catalytically active component is often intensified further by crystallisation enthalpy. The materials normally used as support materials (for example carbon, etc.) generally impart insufficient immobilisation of the catalytically active component as a result of their nonpolar nature. Electrical conductivity can thus also be negatively influenced under some circumstances.
Lastly, a further drawback of the support catalysts known from the prior art that should be mentioned is that they are not always sufficiently chemically and/or mechanically stable under the partly extreme catalysis conditions to be used for sufficiently long lifetimes.